Fluid type lenses, or liquid lenses, have a number of well known properties that make such lenses suitable for medical applications, e.g. for invasive medical instruments for imaging or treatment. For applications requiring high quality performance of the lens, the fluid is often enclosed in a rigid container. However, when exposed to large thermal variations, e.g. the heating of up to or more than 100° C. as required for disinfection of medical instruments, such lenses suffer from the fact that the pressure inside the lens increases due to thermal volume expansion of the fluid. E.g. a water lens with a diameter of 2 mm and a thickness of 1 mm will expand its thickness by about 0.01 mm when exposed to a temperature rise of 50° C. These effects can lead to permanent damage of the lens which therefore in practice excludes such lenses for use more than once.
EP 1 736 802 A2 describes a liquid lens in which internal pressure in the lens due to thermal expansion is remedied by introducing a volume of gas in contact with the liquid of the lens. The gas will then be compressed during high temperature, thus serving to limit the pressure in the fluid lens. However, this solution has the disadvantage that over time the gas molecules will distribute throughout the lens due to diffusion into the fluids that complies the lens. Further, it is a disadvantage of the mentioned lens that the gas container is positioned projecting outward in a radial direction thus increasing the total size of the lens in a dimension perpendicular to the paths of light through the lens.
WO 2004/099847 describes a liquid lens with expansible joints arranged in connection with one transparent end of the container enclosing the liquid. Hereby the transparent end can move, thereby increasing the container volume and thus releasing the pressure inside the lens caused by thermal variation. A disadvantage of such lenses is that optical quality is compromised, because the expansible joints will to some extent allow the transparent window of the lens to tilt compared to the rest of the lens, thereby influencing the optical properties of the lens.